Documentation improvements

[ttaiv]

• The documentation related to exercise development should be migrated from the old development repository. See the documentation in directory doc/exercise_design here. Some relevant files might also be in just the directory doc/. Also see an issue about this in the old development repo.
• Should we in the README of this repo more clearly state what modifications/additions we have made compared to OpenDSA repository?
• Directory JSAV/build contains multiple different builds of JSAV library. Should these be somehow documented? Or are these even necessary anymore? At least when doing a global search across the files of this repo, only file JSAV/build/JSAV-min.js seems to be referenced in the exercise html files.